In audio signal recordings, efforts have been made to modify the pitch and playing rate of sound signals in specific audio applications. For instance, modifications have been attempted in various applications such as in the use of a sampling synthesizer, a harmonizer, a vocoder, a language learning machine, a telephone answering machine, and software for computer synthesized music. When modification of human vocal signals is desired, a compression technique has been used to modify the sound signals according to the pitch of the singer to adjust the amplitude of the signals. In general, the modification range of the amplitude of an adjustable input sound signal is within an octave. The sound signals can be adjusted in a total of 24 halftones including 12 descending halftones and 12 ascending halftones. The modification must match the demand for the real time handling of data by relatively simple hardware design. It must also avoid any detectable distortions of the sound.
Traditionally, a segregation and splicing method utilizing resampling and formatting for the modification of sound signals has been adopted. However, this modification method produces an unacceptable level of sound distortion. The technique of resampling centers on changing the sampling frequency such that it not only changes the amplitude of the sound signal but also changes the signal length and the shape of the format envelope. In order to maintain the original signal length, other workers have performed the compression and expansion technique after resampling of the sound signals. However, these compression/expansion steps frequently produce short durations of pop noise. Furthermore, the changing of the shape of the format envelope produces high pitch noise. The segregation/splicing method utilizes a linear prediction filter and Fourier transformation to maintain the shape of the format, however, the calculation steps required are very extensive. Still other workers have utilized oscillators and filter banks for the modification of sound pitch. These methods produce low frequency and high frequency noises and furthermore, require multiple steps of calculation.
It is therefore an object of the present invention to produce a method for modification of the pitch and playing rate of sound signals that does not have the shortcomings of the prior art methods.
It is another object of the present invention to provide a method for modification of the pitch and playing rate of sound signals by calculating the mean absolute error of the sound signals for the determination of an optimum splicing point.
It is a further object of the present invention to provide a method for modification of the pitch and playing rate of sound signals by calculating the mean absolute error of the signals by incorporating a block binary search method.